1. Field of the Invention
This invention relates to optical recording disks including magneto-optic recording disks, phase change type optical recording disks, and pit-formation type optical recording disks.
2. Prior Art
Optical recording disks each have a recording layer formed on a substrate, and are generally recorded and reproduced by directing a laser beam or similar light through the substrate. Therefore, the substrates used are formed of transparent materials, for example, glass and resins. For a weight reduction and ease of formation of tracking grooves and pits, conventional optical recording disks use resinous substrates with polymethyl methacrylate and polycarbonate being frequently employed.
Among them, polymethyl methacrylate has excellent optical properties including a low coefficient of optical elasticity and a very low double refraction or birefringence, but is poor in dimensional stability due to high water absorption and low heat resistance. Therefore, polycarbonates having low water absorption and relatively high heat resistance are used most often.